Can you use leftover mortar mix to set posts?

Using leftover mortar mix to set posts is not recommended. Mortar is designed for bonding masonry materials and may not provide the necessary strength and drainage properties needed for setting posts. Instead, it's better to use concrete mix, which is specifically formulated for anchoring posts securely in the ground. If you have leftover mortar, consider using it for its intended purpose, such as repairing brick or stone work.